Locking devices of this kind designed as the heel portion of a safety ski binding known from DE 38 08 643 C1. They comprise a housing which is fastened to the ski and at which a retainer member biased in forward direction by a spring assembly is guided so as to be displaceable forwardly in the longitudinal direction of the ski. In a front end area of the retainer member, a hold-down member is supported for pivoting about a transverse axis. The spring assembly which biases the retainer member in forward direction is designed such that it also loads the hold-down member in the sense of forward downward turning. Upon fastening of a ski boot, the rear sole portion thereof is pressed forward by a lower projection formed at the hold-down member, while a hook-shaped projection provided further upwards at the hold-down member presses the rear sole portion downwardly. Behind the hold-down member in the housing of the locking device there is a rod which is guided for longitudinal displacement and biased in forward direction so that its front end rests against a back surface of the hold-down member. Further backwards, the rod is formed with a cam, and a switch in the power circuit of an electromagnet is associated with the cam for unlocking the locking device. In case of a fall towards the front, the hold-down member is pivoted upwards and backwards by the rear sole portion which moves in upward direction so that the switch is actuated by way of the rod and its cam, whereby the electromagnet is supplied with power. Consequently, an armature of the magnet is pulled forward, hitting against the rear end of a longitudinally displaceable pawl which is biased to the rear and has a rearwardly tapering conical portion engaging between two roller bodies, acting as locking members, which it normally maintains in a locking position in which they in turn retain a support member in an inoperative position in the housing. As soon as the pawl is propelled forward by the armature, the two locking members leave their locking position, thereby releasing the support member. The support member, on which the rear end of the spring assembly rests, moves backwards under the pressure of the spring assembly into a retracted position. In this manner the bias of the spring assembly is reduced so that the retainer member and the hold-down member supported on the same both move backwards, releasing the rear sole portion of the ski boot.
This known locking device works perfectly if the dimensions of the electromagnet and its current supply are of such magnitude that its armature will hit the rear end of the pawl with considerable kinetic energy, thus overcoming the friction which is opposed to the required forwardly directed movement of the pawl clamped between the two locking members. This friction depends on the bias of the spring assembly converted by the hold-down member into forwardly and downwardly directed forces acting on the rear sole portion. The armature must have a considerable inert mass in order for the kinetic energy, needed for overcoming the friction, to be built up in the armature. If this inert mass is to be prevented from inadvertently releasing the safety ski binding under certain skiing conditions, the armature must be biased rearwardly by its own spring and, as a consequence, the power requirement of the electromagnet becomes even greater.